In the pressure treatment of web-shaped materials, e.g. paper or cardboard, there is often used a rotatable roll which bears against another rotatable roll, whereby a pressure is created in the nip between the two rolls. The pressure nip is used, for example, for dewatering a web material, smoothing a web material, or for pressing two or more layers of a composite web material together. Examples of such arrangements of rolls are calender rolls and roll presses.
A roll conventionally comprises an annular roll shell which rotates about a central axis. The roll is normally suspended on a shaft, the shaft extending out of the roll at both ends thereof and being mounted in bearings on a suspension arm at each end. Such a suspension arm may comprise a straight-armed lever, wherein the roll shaft is mounted in bearings at a first location on said suspension arm, about a first end of the arm. The suspension arm itself may be mounted in bearings on a support structure at a second, intermediate location of the arm. Finally, a balancing force may be applied to the suspension arm at a third location thereof, about a second end of the arm. Typically, the suspension arm is suspended in the support at said second location, between said first and third location, somewhere along the arm. The purpose of the balancing force which is applied at the third location is to press the roll against an adjacent roll, in order to form a pressure nip. The size of the balancing force which is needed is typically calculated from the length of the two lever arms, the weight of the roll, and the linear load in the nip.
However, a standard design, such as the above, often exhibits problems due to vibrations in the roll during operation, which can lead to quality deficiencies of the web being produced or even premature wear or break down of the machine. The vibrations are produced for a variety of reasons, e.g. speed variations of the driving roll or the gear box, a non-round surface of the roll, varying hardness/thickness of the coating of a rubber coated roll, varying liquid content of a press felt passing through the nip, varying thickness of the paper web, etc. This implies that the theoretical force balancing system which is used to determine the configuration of the nip is not a true representation of the actual forces in the system.
There have been many suggestions as to how the vibration problems in connection with the operation of such rolls could be solved. Most of the suggestions relate to different devices which are directed to damping the vibrations. Such devices are for example shown in U.S. Pat. Nos. 3,512,475; 5,730,692; 4,910,842; 5,081,759; DE 196 52 769 and EP-B1-0 268 769.
DE 42 32 920 discloses a method directed to avoiding the formation of vibrations, rather than damping the vibrations. However, this method does not primarily relate to eliminating vibrations in relation to rolls for producing a paper web, wherein such paper webs are extremely thin, e.g. 0.1-3.0 mm. Moreover, this method does not focus on the suspension of the rolls.